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frequency domain distortion, some recordings have high levels of all frequencies
AndyH-ha
post Dec 7 2012, 01:06
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Has anyone run into this kind of distortion problem? Most especially, is there a solution?

The levels are good, no clipping. Spectral view shows that fairly high, and pretty constant, levels of unnatural sound is present from somewhere around 2kHz to 4kHz, all the way to the Nyquist limit. In reality, the distortion might start somewhere near 0Hz but I can't really tell from what I see.

The recordings are spoken audio coming from cassette. This distortion is only present when speech is present but there is more background noise than usual during the pauses. Most of the real speech content is 4kHz and less, but there is a little up to about 10kHz. When this (intermittent) problem occurs, it is there from the first word on the cassette to the last (in the computer recording). It is not on the cassette. The first time it happened I re-recorded without difficulty. There are now a few more recordings I must do over. I have not yet looked at those cassettes but the problem effected only one side of each. I much doubt that the cassettes, or the recordings on them, are at fault.

When I record, I monitor at the tape deck headphone out. Everything sounded proper there on all of the cassettes. Often, but perhaps not always, I listen to the headphones from time to time as the cassette is playing, just to be sure things are still good, but I suppose that is irrelevant since when the problem occurred it occupied the entire recording session. It never started sometimes during the recording.

What is new is the computer. Actually it is a used corporate machine running WinXP professional that I acquired recently. This newer used computer has a Intel Core 2 Duo running at 3GHz, 2GB of RAM, and two SATA hard drives, running up-to-date WinXP Pro. I installed a Echo Mia soundcard, which I had been using for many years in a much older, slower Win98 computer, with a current (probably several years old, being for WinXP) Echo driver.

The machine isn't allowed to do anything automatic that I can find. I disabled the anti-virus, firewall, task scheduling, etc. Besides, all my experience with excess background activity says missing samples will be the result of too few resources, not frequency domain distortion.

The soundcard is using Echo's Purewave driver, which Echo says addresses the soundcard hardware directly, bypassing all Windows manipulations. I could not use that driver on Win98, but everything worked quite well there. For this latest set of cassettes, where the problem surfaced, I switched from recording at 16 bit to 32 bit (Cool Edit 2000). Prior to the switch to 32 bit, I had recorded more than 20 cassettes on the same setup at 16 bit with no problems.

The weird behavior might be related to the higher bit depth, the problem first appeared on the first cassette, recorded at 32 bit, but since it does not effect most recording sessions, that can't be the complete answer -- if it is relevant at all.
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AndyH-ha
post Dec 7 2012, 06:53
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The soundcard is 24 bit. Like every other 24 bit soundcard, its dynamic range is less than 24 bit, but it works at 24 bit. Thermal properties of basic electronic components prevent converters getting very near 24 bit without cryogenic cooling.

This can not be reproduced at will. So far, no cassette has made a distorted recording twice. It might go away completely if I only recorded at 16 bit but I've been using the soundcard at both 16 and 24 bit (32 bit floating point is the file format) at will, for years with no problem. If the problem resides in the soundcard, and not in the computer or WinXP, it is a problem that just developed.

Yes 16 bit is plenty for the material. It is probably plenty for the processing too but the relative amount of quantization error is markedly smaller for each step when in floating point format, so I choose to use it.

Recording at 16 bit, and converting to 32 bit before other processing, may take less extra time than re-recording the occasional piece, if the higher recording bit depth actually has anything to do with the problem. In terms of actual total time, which is more efficient would depend on how frequently it happens. I already understand all these obvious things, I am just hoping for some matching experience or insight to the actual malady.
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xTobix
post Dec 8 2012, 06:51
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QUOTE (AndyH-ha @ Dec 7 2012, 06:53) *
The soundcard is 24 bit. Like every other 24 bit soundcard, its dynamic range is less than 24 bit, but it works at 24 bit. Thermal properties of basic electronic components prevent converters getting very near 24 bit without cryogenic cooling.

Agreed - I cool my detectors at work to -162 deg C ;-) Maybe I should do that with my DAC too :-D

QUOTE (AndyH-ha @ Dec 7 2012, 06:53) *
This cannot be reproduced at will. So far, no cassette has made a distorted recording twice. It might go away completely if I only recorded at 16 bit but I've been using the soundcard at both 16 and 24 bit (32 bit floating point is the file format) at will, for years with no problem. If the problem resides in the soundcard, and not in the computer or WinXP, it is a problem that just developed.

If hardware and driver software support 24 bits and you have done it before then I doubt this is the source of the noise.

QUOTE (AndyH-ha @ Dec 7 2012, 06:53) *
Yes 16 bit is plenty for the material. It is probably plenty for the processing too but the relative amount of quantization error is markedly smaller for each step when in floating point format, so I choose to use it.

I don't know how much post processing you are going to apply. I would say with a regular cassette player and tapes of some age you are somewhere between 10-14 bit before it disappears in the noise floor. However, this is entirely up too you.
Since it comes and goes this has to be some noise source close or inside your PC. My first attempts to tackle this would be in order:

1) Run all you fans including the one in the power supply itself of an external power supply/battery. ( you can use a second PC power supply just for the fans!

2) turn off Bluetooth, WiFi, disconnect LAN, and disable all other soundcards (onboard in bios)

3) Remove all drives and cards not essential for recording from the power supply and the data bus (i.e. 2nd/3rd HDD, CD-ROM, DVD, Floppy drives, and all other PCI cards). Also all external devices attached on USB, RS232, LPT, firewire, ... (printer, drives, ect.)
4) Take some aluminum kitchen foil and try to shield your entire soundcard (make a sort of sleeve that slides over the pci card). You have to be very careful that you do not shortcut anything (you could use some plastic sleeve first) and the aluminum must be in good contact with ground!
5) Terminate/Short cut all unused internal and external inputs and outputs with a wire or some plug.

To me, it seem like a fan is turning on and off or a drive or something else. Fans are nasty noise sources by their nature (this is true for DC and PWM driven fans!).

Let me know if this solved/change things.
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